1. Abstract
Human activities have often led to degradation of the world’s land resources. The global assessment of human-induced soil degradation (GLASOD) has shown that degradation results mainly from erosion and salinization, which are discussed in this review. Land conservation and rehabilitation of deserted or saline land are essential parts of sustainable agricultural development (OLDEMAN ET AL., 1990). For this revegegation with shrubs, inoculation with mycorrhiza, wind brakes, strip cropping and mulching to combat desertification as well as revegetation with halophytes to combat salinization play an important role. These practices are widely available but are not used for many reasons (DREGNE, 1995). That is why many international conventions were formed to identify, promote and implement effective responses to land degradation in dryland areas to combat degradation. In conclusion a sustainable development in arid countries depends on combating degradation in the nearest future to ensure the economic and social wellbeing in these countries (UNCCD ED., 2002).

2. Introduction
The soil is a three-dimensional body that forms the uppermost layer of the earth’s crust. It supports terrestrial life, filters water, biodegrades pollutants and moderates gaseous exchange between ecosystems and the atmosphere. Additionally, it is the primary medium for food production (LAL ET AL., 2004).
Arid areas are characterized by low rainfall (< 250 mm) and high rates of evaporation. They occupy 41% of the Earth’s land area and are home to more than 2 billion people. The soils in arid areas are continuously threatened by human activity, which includes the elimination of plant cover, overgrazing, deforestation and poor irrigation strategies. With the reduction of soil fertility and elimination of plant cover, land degradation is set in motion (CUANCA ET AL., 1998), which results in a reduction or loss of biological productivity because of vegetation degradation, wind and water erosion, salinization and soil compaction (UNEP ED., 2006). All in all degradation has already affected 1900 million hectares of land globally (DE MAN ET AL., 2007). It is further estimated by BRIDGET ET AL. (2001) that the annual economic costs of desertification alone exceed 42 billion US$. Additionally each year over 14 million acres of productive lands are oversalted because of improper management of water (BRIDGES ET AL., 2001). Therefore, desertification and salinization are the most important types of land degradation in arid areas. That’s why in this review strategies to combat these two forms of land degradation are introduced.

3. Forms of degradation in arid climate
Land degradation in drylands is defined as reduction or the loss of biological or economic productivity of drylands. Land degradation includes wind erosion, water erosion, chemical and physical deterioration and severe degradation. The global assessment of human-induced soil degradation (GLASOD) has shown that the damage results mainly from erosion and salinization (OLDEMAN ET AL., 1990).
Land degradation is caused by disturbance of the soil balance with its environment either by natural or anthropogenic influence. The driving forces of anthropogenic influences include the increase in population and human needs which lead to farmland expansion for production of cash crops and to conflicts over land and water use (UNEP ED., 2006). Additionally, poor irrigation practices often result in salinization, a form of chemical degradation (UNCCD ED., 2002). Natural aspects include climate change and limited water resources due to low or seasonal rainfall which lead to thin plant cover and low content of organic matter in the soil. Climate change contains the increase of average temperatures and extreme weather conditions, like storms and heat periods. These changes go along with a decline in soil quality which results in a reduction in biomass productivity and environment moderation capacity (LAL ET AL., 2004). Low vegetation cover, high temperatures and erosion lead to desertification, a form of physical degradation. The result is that every year 84 billion tones of productive topsoil are lost worldwide through degradation (DE MAN ET AL., 2007). In arid climate between 10 and 20% of drylands are degraded (UNEP ED., 2006). Altogether degradation has already affected 1900 million hectares of land globally. Because erosion has made land unproductive, nearly one third of the world’s cropland has been abandoned in the past 40 years (UNEP ED., 2006). As a result desertification and salinization are key driver of economic loss and stagnation, poverty and insecurity (DE MAN ET AL., 2007).

Desertification
Desertification is according to ZHAO ET AL. (2007) “the gradual spread of a desert into adjoining lands”. It is one of the main types of land degradation in arid areas. The United Nations Conference on Desertification 1977 defined that “desertification is the diminution or destruction of the biological potential of the land, ultimately leading to desert-like conditions”. The factors responsible for desertification are livestock overgrazing, deforestation and elimination of plant cover in general. Degradation or destruction of plant cover leads to a disruption of the carbon cycle. The soil organic matter decreases and the soil’s physical properties are thus degraded. These processes are nearly irreversible because a natural recovery of structural stability, below-ground microorganisms, organic carbon content or bulk density is mostly not observed (ALBASALADEJO ET AL., 1998).
Additionally wind erosion increases with the removal of plant cover. In particular, desertification of sandy areas driven by wind erosion often result in poor and fragile soil and low land productivity, which can degrade the human living environment and impede socioeconomic development (ZHAO ET AL., 2007). The movement of sand is harmful because the areas, where the soil is removed and where it is deposited, can lose their productivity. Additionally, the sandy wind causes damages to crops and trees (GUPTA, RAINA, 1994). That is the reason why in this work the main stress is on wind erosion.

Salinization
Besides desertification salinization is considered to be one of the most important land degradation processes. On a world scale there is an area of around 380 million hectares where production is severely restricted by salinity. These areas occur predominantly in arid regions where evaporation exceeds precipitation (UNCCD ED., 2002). Because of improper management of water, like poor irrigation and drainage practices, each year over 14 million acres of productive lands are oversalted (RAVINDRAN ET AL., 2007). The mechanism behind is, that the salt percolates with the irrigation water in the soil. When the water with the diluted salts evaporates the salts are taken to the surface of the soil. As a result the United Nations Environment Program (UNEP) estimates that approximately 20% of agricultural land and 50% of cropland in the world is salt stressed. The salt accumulation in soils hinders the growth of most crops because it leads to dehydration of cells, nutrient antagonism of Potassium and toxic ion effects of Sodium. Therefore, salinization causes poor productivity in farming in many arid regions (LAMBERS ET AL., 1998).

4. Strategies for soil recovery
Because desertification and salinization are very important forms of land degradation worldwide, soil recovery strategies are searched to ensure sustainable management of arid lands. In the following strategies for desertification and salinization recovery are introduced.

Strategies to combat desertification with main stress on wind erosion
The aim of soil conservation is according to MORGAN (2005) to “reduce erosion to a level at which the maximum sustainable level of agricultural production or grazing can be obtained from an area of land without unacceptable environmental change”. Here revegetation with shrubs, inoculation with mycorrhiza, wind brakes, strip cropping and mulching as strategies for wind erosion control are presented. These strategies to combat wind erosion have the cover of the soil with plants or plant residues in common.

4.1.1. Revegetation with shrubs
In the case of revegetation pioneer species are established to give immediate cover, to improve the soil and to permit native species to come in. Generally a mix of plant species is required because it is impossible to predict the success of one species in marginal environments. When developing a plant for revegetating an area, soil analyses should be carried out to establish pH, nutrient levels, moisture status, salinity levels and the presence of toxic ions. These parameters influence the range of species that will grow (MORGAN, 2005).
Because shrubs are adapted to arid climate and support biodiversity as well as soil moisture and organic matter, revegetation of shrubs is one possibility to regenerate degraded land and decrease erosion.
ZHAO ET AL. (2007) found out that fine sand content and bulk density are lower, while clay and silt content, soil moisture, organic matter, total N and available P are higher under the canopies of shrubs than in open spaces. As a result shrubs improve soil fertility as well as soil texture. Additionally, the increase in fine soil particles can lead to decreased bulk density and increased soil moisture.
Furthermore shrubs had significant effects on the herbaceous community under the canopy. The herbaceous community, which is usually restricted to fixed sand dunes, not only survived under shrub canopy in shifting sand dunes, but also had higher plant densities, plant height, cover and aboveground biomass. So they assumed that shrubs provide protection to herbaceous plants against windblown sand.
Additionally, plant cover slows local climate change by absorbing carbon form the direct impact of sunlight and generating atmospheric moisture.
Overall shrub cover can reduce wind velocity and trap soil particles in transport. As a result sand dunes can be fixed by shrubs.

4.1.2. Inoculation of plants with mycorrhiza fungi
It is widely known that mycorrhiza fungi play an important part for a successful implementation of revegetation efforts. The importance especially of vesicular-arbuscular mycorrhiza (VAM) for the increased phosphate uptake by plants in P-deficiency soils is well recognized. The primary mechanism is considered to be the hyphal transport of available soil phosphate to the plants. Additionally, VAM infection increases water uptake and drought tolerance of several plant species (SKUJNS, ALLEN, 1986). As a result of the symbioses with mycorrhiza plants are able to get nutrients and water, which results in a better plant cover of the surface. With this the risk of wind erosion is decreased.

4.1.3. Wind brakes
To plant windbreaks is another method to control wind erosion. Windbreaks are multirows of trees and shrubs, planted perpendicular to the wind direction to protect soil and crops from wind erosion and dry winds (GUPTA, RAINA, 1994).
Array cropping is one form of a windbreak. The rows of trees are planted 8 to 10 meters apart. Within these trees a crop can grow (MORGAN, 2005).
When selecting which plant species to use in wind brakes, high drought resistance, fast growth and ease of seedling production are important factors. The effects of competition on crop productivity should be minimized by planting tree species with low water requirements and by using pruning to limit tree transpiration. Acacia, Eucalyptus or Prosopis juliflora are trees which are often used in alley cropping. Careful consideration needs to be given when Prosopis juliflora is introduced into regions because this plant spreads its roots vigorously, often choking off the other species which grow nearby.
Although logging and harvesting is not the main objective for planting wind brake forests, mixed forests which can also be used in bee-keeping and as feed for livestock, which can ease the cost of planting the forest (TERAKAWA, 1996).

4.1.4. Strip cropping
With strip cropping, row crops and protection-effective crops are grown in alternating strips aligned on the contour or perpendicular to the wind. Erosion is largely limited to the row-crop strips and soil removed from these is trapped within and behind the next strip downwind, which is generally planted with legumes or a grass crop.
Strip-cropping is best suited in well drained soils, because the reduction in runoff velocity, combined with the low rate of infiltration on poorly drained soil, can result in water logging and standing water. The main disadvantage of strip cropping is the need to farm small areas, which limits the kind of machinery that can be used (MORGAN, 2005).

4.1.5. Mulching
Mulching is the cover of the soil with crop residues such as straw, maize stalks, palm fronds, standing stubble or plastic material (JENSEN, 1996). The cover reduces the velocity of wind and prevents water loss. It is most useful as an alternative to cover crops in dry areas where insufficient rain prevents the establishment of a ground cover before onset of heavy rain or strong winds, or where a cover crop competes for moisture with the main crop (MORGAN, 2005). With mulching the soil moisture can be conserved and the soil temperature be moderated. Additionally, weeds are suppressed.
In conclusion with revegetation wind erosion can be reduced. The UNEP estimates that an effective 20-year global effort to combat desertification in general would cost 10-22 billion US$ per year whereas desertification decreases the income by 42 billion US$ per year (UNCCD ED., 2002). The investment in combating desertification and especially in combating wind erosion is an important way to ensure sustainability for future generations.

Strategy to combat salinization by revegation with Halophytes
With revegetation and a better management of water the problem of salinization can be solved. The revegetation of a salt-affected land with halophytes is an example of phytoremediation. Halophytes grow naturally on the coastal and inland saline areas such as salt marshes and salt deserts and survive salt concentrations equal to or greater than that of seawater. They use the controlled uptake of Sodium into their cell vacuoles to drive water into the plant against a low external water potential. Through their capacity for salt uptake, halophytes can reduce the salt content of soil over time. Unfortunately the reduction of salt concentration in the soil is a slow process depending on the salt concentration in the soil and the salt accumulation during the growth of the halophytes (BARETT-LENNARD, 2002).
Many halophyte plants are not only salt tolerant but can grow in poorly drained soil. Although they are grown on these poor and salty soils halophytes such as the salt bush Atriplex nummularia can outperform conventional crops in yield and water use efficiency (GLENN, BROWN, 1999). Additionally, the salt bush Artiplex lentiformis has been reported to produce good quality forage on salt lands. So it can be used to feed livestock (GUPTA, ARYA, 1995). Altogether halophytes can be used for revegetation of salty soils. The salinization decreases with the time and other crops can be grown afterwards. To hinder new salinization of arable land the management of water has to be improved. For this drip irrigation can be used. Drip irrigation increases the irrigation efficiency up to 95% (EVET ET AL., 2000). In combination with drip irrigation mulching can be used to decrease the evaporation of salt containing water (MORGAN, 2005).

International conventions to combat desertification
The presented strategies to control wind erosion and salinization are widely available but are not used for many reasons. Absence of a food crisis in developed countries and the perception that no real problem exists are the most important reasons. A third reason is that degradation control is not cost effective, except for controlling salinity and compaction (DREGNE, 1995).
Therefore, many international conventions were formed to identify, promote and implement effective responses to land degradation in dryland areas to combat degradation (UNCCD ED., 2002). Within these the Word Conservation Strategy (WCS), the Rio Earth Summit as well as the development of the UNCCD are very important international conventions.
The WCS is a plan of action for governments and public bodies to combat degradation around the world. Following the WSC many nations prepared national conservation strategies to address environmental degradation and resource depletion. The sustainable development in arid countries can be achieved through the improvement of environmental law and policy aimed at land degradation management (HANNAM, 2000).
In 1992 the Rio Earth Summit recommended that the United Nations General Assembly establish an Intergovernmental Negotiating Committee for a Convention to Combat Desertification in Countries Experiencing Serious Drought or Desertification (INCD). The United Nations Convention to Combat Desertification (UNCCD) was adopted in June 1994 in Paris (UNCCD ED., 2002). The goal of UNCCD is the “identification, promotion and implementation of effective responses to land degradation in dryland areas”. The central focus of the UNCCD is to address the underlying causes of desertification and drought and to prevent and reverse the problems of land degradation in arid, semi-arid and dry sub-humid regions through the instruments of National, Sub-regional and Regional Action Programs (NAPs, SPAPs and RAPs). To combat this effort, article 21 of the Convention established the Global Mechanism (GM) whose main mandate is “to increase the effectiveness and efficacy of existing financial mechanisms and to promote actions leading to the mobilization and channeling of substantial financial resources, including for the transfer of technology to affect developing countries”. Additionally, the LADA (Land Degradation Assessment in Drylands) process started with a request from the UNCCD to look in more detail at land degradation issues and desertification and to build a scientific basis for land degradation estimates and assessments (FAO ED., 2002).
The presented international conventions realized that land degradation and alleviating poverty go hand in hand (UNCCD ED., 2002).
Overall a sustainable development in arid countries depends on combating degradation in the nearest future to ensure the economic and social wellbeing in these countries.

5. Conclusions
Revegetation with shrubs, wind brakes, strip cropping or mulching to combat desertification and revegetation with halophytes to combat salinization are very effective and widely available, but are as presented not used for many reasons. For the future it is important to increase the productivity of land, because of the growing population and the increasing land degradation in general. An important aspect concerning sustainable development in arid countries is that the economic and social lost of land degradation are higher than the costs to combat degradation. Therefore, the affected countries should be convinced to combat degradation within international conventions in the nearest future to ensure their economic and social wellbeing as well as the sustainability for future generations (UNCCD ED., 2002).